日本九州俯冲带b值成像及其构造意义 *

日本九州俯冲带是菲律宾海板块与欧亚板块汇聚边界上一个独具特色的区域, 也是研究俯冲带内板块构造作用的理想场所。为了解该俯冲带内的板间应力状态和相互作用, 本研究利用震源深度大于20km的97251个地震事件, 通过b值计算详细刻画了该俯冲板片上表面以及垂直海沟走向的剖面特征。结果发现, b值表现出明显的空间变化, 整体上沿南海海槽和琉球海沟从东北往西南方向逐渐增大, 同时在俯冲的九州-帕劳海脊上存在显著的低值区。从b值与应力的负相关性推断, 进入俯冲带的海脊以及海脊东北侧的四国海盆洋壳与俯冲带上覆板片耦合作用较强; 而在海脊西南侧, 俯冲带内汇聚板片的耦合作用相对较弱。究其原因, 本文认为九州-帕劳海脊两侧俯冲洋壳在形成时代和汇聚速率上的差异起着重要作用。对于九州-帕劳海脊来说, 俯冲带浅部的低b值区主要是由于隆起的海脊增强了与上覆板块的耦合作用。随着俯冲深度的增加和俯冲板片倾角的急剧变陡, 沿海脊可能发生了板片撕裂, 从而释放了海脊与上覆板片间的挤压-剪切应力, 使耦合程度大大减弱。

The b-value mapping in the Kyushu subduction zone and its tectonic significance

The Kyushu subduction zone is a unique region on the convergent boundary between the Philippine Sea Plate and Eurasian Plate. The zone is an ideal place to study plate interaction within the subduction zone. To understand the stress state and interaction characteristics between plates in the subduction zone in this region, we use 97251 earthquake events beneath 20-km depth in the region to delineate the upper surface of the subduction plate and the section of the vertical trench direction by detailing b-value. We found that the b-value in this area has significant spatial variation, which gradually increases from the northeast to the southwest along the Nankai Trough and Ryukyu Trench. There is also a significant low b-value zone on the subducted Kyushu-Palau Ridge. It is inferred from the negative correlation between the b-value and stress that the ridge has entered the subduction zone, and that the Shikoku Basin ocean crust on the northeast side of the ridge is strongly coupled with the overlying slab of the subduction zone, while the coupling effect of the convergent plates in the subduction zone on the southwest side of the ridge is relatively weak. We believe that the formation times and convergence rates of subduction ocean crusts on both sides of the Kyushu-Palau Ridge played important roles. For the Kyushu-Palau Ridge, the low b-value zone in the shallow part of the subduction zone is mainly due to enhanced coupling between the towering ridge and overlying plate. With the increase of the subduction depth and the steepening of the inclination of the subducting plate, a plate tear along the ridge may occur, thereby releasing the compression-shear stress between the ridge and overlying plate, and causing the degree of coupling to be greatly weakened.